The present disclosure relates generally to a trip system for a circuit breaker, and particularly to a system and method for controlling the mechanical stress at a thermal-magnetic trip unit of a circuit breaker.
Electrical circuit breakers may employ a variety of trip systems for sensing an electrical current and for initiating a tripping action at the circuit breaker, including bimetallic, magnetic, and thermal/magnetic trip units. Magnetic trip units may include c-shaped magnets, oil-filled dashpots, coil-type solenoids, and the like. Thermal trip units may include bimetals, shape memory alloys, and the like. Each phase of a multi-phase circuit breaker has a separate current sensor for that phase, which interfaces with an operating mechanism through a common trip bar and latch arrangement. Motion at an individual trip unit is transferred to the common trip bar, which is then driven to release a latch coupled to the operating mechanism, thereby resulting in a trip condition. To properly set the trip unit tripping characteristics, circuit breaker manufacturing processes employ a calibration routine that coordinates the responsiveness of the trip unit to an electrical current and adjusts for dimensional variations and tolerances among and between the circuit breaker components. One such calibration routine involves the adjustment of a calibration screw that biases the bimetal to an initial position. However, during a short circuit condition, excessive resistance heating or magnetic repulsion forces may result in excessive deflection and cause mechanical stress at the trip unit, which may have the drawback of introducing variation into the calibration setting. Shunting contacts or flux shunts may be employed to redirect the electrical current or magnetic flux, respectively, under a short circuit condition, thereby reducing the resultant mechanical stress seen at the trip unit, but the shunting contacts and flux shunt may not be sufficient to prevent an overstress condition at the trip unit under a high short circuit condition. Accordingly, there is a need in the art for a trip system for a circuit breaker that overcomes these drawbacks.
In one embodiment, a trip system for a circuit breaker includes a current sensor and a stop surface, the current sensor having a contact surface, a first end that is supported, and a second end with a degree of freedom. The current sensor, arranged for receiving an electric current, undergoes a first deflection in response to a first current and a second deflection in response to a second current, the first deflection resulting in clearance between the contact surface and the stop surface, and the second deflection resulting in contact between the contact surface and the stop surface.
In another embodiment, a method for controlling the mechanical stress at a current sensor assembly of a circuit breaker is disclosed. One end of a current sensor of the current sensor assembly is restrained and the current sensor energized. The unrestrained portion of the energized current sensor is permitted to deflect freely, but prevented from deflecting freely prior to the mechanical stress level at the current sensor reaching the mechanical yield point stress of the current sensor material.